1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems of the traction type.
2. Description of the Prior Art
It is common in traction elevator systems to set the brake to restrain rotation of the traction or drive sheave when the elevator car is stopped at floor level with its door open to permit passenger transfers. The floor leveling device may be maintained active during passenger transfer to permit stretch of cable leveling during which the brake is partially released to allow the elevator drive motor to overcome the partial restraint of the brake to maintain the elevator car at floor level as the load in the elevator increases or decreases. When a run is to be made the leveling device is deactivated, door closure is initiated, and the elevator car is held stationary with the brake until its door reaches the closed position. The brake is then released and the acceleration pattern applied to the drive motor to move the car away from the floor. With a geared traction elevator, the gear aids the brake in restraining sheave movement when the car is stopped and the brake may be relatively small, compared with the brake required in a gearless elevator system where the brake is "right on the cables". The inductive lag in a small brake is relatively short and thus the car may be started without appreciable delay after the door reaches the closed position. The large brake on a gearless elevator has a substantial time constant, which may be in the range of 1/2 to 2 seconds, and thus the car is delayed in leaving the floor level, waiting for full brake release. Also, depending upon car direction and unbalanced load, a rough start or "bump" may be experienced as the drive motor builds up armature current to compensate for the unbalanced load, and considerable effort has been expended to improve the starts for all load conditions.